I would first locate the regulator, it will be a finned box about 3 to 4" square. The center wire should go back to the ignition switch or possibly start solenoid. While running, use your red probe on that wire, black on ground, on a meter set to DC voltage and see what you have, should be 14-14.8 volts on mid throttle. If nothing, then your regulator is most likely bad. Also check the wiring for any fuses, make sure those are all good and not blown, actually do that first. You can also check the other two wires going into the regulator, red probe on wire, black on ground, meter set to AC voltage. I m not sure the exact voltage you should get on an Onan, I know my Tecumsehs put out around 20-28 volts per side.